Game crashes at startup - downloader issue?

I'm trying to install XIV on my PC, but when the 45gb download has finished, the game crashes with multiple ffxiv_dx11.exe errors (in Japanese).

I've read previous threads about this and I've done the following already:
-reinstalled the game
-installed the latest graphics driver version
-installed DirectX 11
Nothing helps.

I've always had trouble with downloading the game itself. It's 45 gigabytes and I have a slow Internet connection, so I often have to cancel the download and resume it later. Could this somehow corrupt the game files?
The XIV downloader is probably the worst I've seen. Sometimes I've downloaded 70% of the game, cancel the download, and it resumes from literally 0%. Same happens if I just close the window instead of clicking "cancel". Downloader has no idea where to pick up. 40 gigabytes of data just disappears? Never had similar issues with Battle.net for example.
I'm wondering if this is causing the errors because everything else should be fine.

PC specs:
Acer Predator G3-710
8GB RAM
Intel i5-6400
250GB SSD drive
Geforce 960 GX
Windows 7 SP1 with updates

Is this just because I'm running on Windows 7?

Official Square source says that it "should" work:

Quote:

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You will likely be able to play FINAL FANTASY XIV on Windows® 7 systems even after the end of support.

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